What R-value do you need for each assembly type? This guide covers recommended values by application and climate zone — with product picks for each scenario.
R-value (thermal resistance) measures how effectively insulation slows heat flow. Higher R-value = better insulation performance. The number tells you how many hours it takes for one BTU of heat to pass through one square foot of material with a one-degree temperature difference.
In practical terms: a wall insulated to R-21 loses heat about 10% more slowly than the same wall at R-19. For an attic, the difference between R-30 and R-49 is substantial — attics are responsible for 25–30% of a home's heat loss, so getting the attic R-value right has the biggest single impact on energy performance.
Standard for interior partitions and older exterior walls. Your maximum batt R-value with 2×4 framing is R-15 (high-density). R-13 is the most common choice — it's code-compliant in most climate zones and fits standard 2×4 cavities perfectly.
| Application | Min. R-Value | Recommended Product |
|---|---|---|
| 2×4 exterior walls, zones 1–3 | R-13 | Fiberglass R-13 · 15" wide |
| 2×4 exterior walls, zones 4–8 | R-15 | Fiberglass R-15 HD · 15" wide |
| Interior walls (sound control) | Any | Mineral Wool AFB · 15" wide |
Standard for modern exterior walls and cold-climate construction. Your batt options are R-19 (full-thickness) and R-21 (high-density). Most energy codes in climate zones 4 and above require at least R-20 for above-grade wood-framed walls, making R-21 the standard choice.
| Application | Min. R-Value | Recommended Product |
|---|---|---|
| 2×6 exterior walls, zones 1–3 | R-19 | Fiberglass R-19 · 15" or 23" wide |
| 2×6 exterior walls, zones 4–8 | R-21 | Fiberglass R-21 HD · 15" or 23" wide |
| Party walls (multi-family, fire-rated) | R-23 | Mineral Wool AFB R-23 · SAFB |
Attics are the highest-priority assembly for insulation upgrades. Heat rises, so an under-insulated attic is your biggest source of thermal loss in winter — and heat gain in summer. Code minimums for attics are higher than walls, and the ROI on attic insulation upgrades is typically the fastest of any assembly type.
| Climate Zone | Example States | Code Min. | Product |
|---|---|---|---|
| Zones 1–2 | FL, HI, TX (south) | R-30 | Fiberglass R-30 |
| Zones 3–4 | GA, VA, OR, WA | R-38 | Fiberglass R-38 |
| Zones 5–8 | NY, MN, CO, AK | R-49 | Fiberglass R-49 |
For attic floor insulation, batt orientation matters. Batts should sit between joists, not compressed. Compression reduces effective R-value — a compressed R-38 batt may only deliver R-30 in practice.
Floor insulation goes between the floor joists above an unconditioned crawl space or garage. The most common product for this application is R-19 fiberglass (15" wide for 16" OC joists), because it's thick enough to fill a standard 2×10 joist bay and includes kraft facing that can be stapled to the framing.
| Application | Typical R-Value | Notes |
|---|---|---|
| Floor over crawl space, zones 1–3 | R-19 | Kraft-faced, staple-up installation |
| Floor over crawl space, zones 4–8 | R-30 | May require double-layer or deep joist |
| Floor over garage (conditioned above) | R-19 to R-30 | Match cavity depth to joist size |
| Between floors (sound control) | Any | Mineral Wool AFB preferred for STC |
For most wall, attic, and floor applications, fiberglass is the cost-effective default. Mineral wool is worth specifying when you need fire resistance (party walls, floor assemblies in multi-family), superior sound control, or moisture tolerance (basements, below-grade walls).
For a full comparison, see our guide on fiberglass vs mineral wool insulation.
Once you've determined the right R-value for each assembly, use our insulation bag calculator to convert square footage into bag counts — with a 10% waste buffer included.
We stock fiberglass batts from R-11 through R-49 and mineral wool in AFB, SAFB, and standard configurations. Same-day quote, next-day delivery nationwide.
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